[wwwdocs] Document that gcc-8 changed the default to -std=gnu17

[Jonathan Wakely]

OK for wwwdocs?

---
 htdocs/gcc-8/changes.html | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/htdocs/gcc-8/changes.html b/htdocs/gcc-8/changes.html
index c329a509..6761fb90 100644
--- a/htdocs/gcc-8/changes.html
+++ b/htdocs/gcc-8/changes.html
@@ -24,6 +24,8 @@ You may also want to check out our
 <!-- .................................................................. -->
 <h2>Caveats</h2>
 <ul>
+  <li>The default mode for C is now <code>-std=gnu17</code> instead of
+        <code>-std=gnu11</code>.</li>
   <li>Support for the obsolete SDB/coff debug info format has been
       <strong>removed</strong>.  The option <code>-gcoff</code> no longer
       does anything.</li>
@@ -536,6 +538,11 @@ $ gcc unclosed-2.c
     </li>
 </ul>
 
+<h3 id="c">C</h3>
+  <ul>
+    <li>The default mode has been changed to <code>-std=gnu17</code>.</li>
+  </ul>
+
 <h3 id="cxx">C++</h3>
 <ul>
   <li>GCC 8 (<code>-fabi-version=12</code>) has a couple of corrections to the calling
-- 
2.46.2